The present invention relates to equipment for monitoring rod pumped oil wells and more particularly to an accelerometer for measuring the acceleration of the rod string. Many oil wells require some means of artificial lift for producing the oil after the initial water or gas drive has been depleted. A conventional method for artificially lifting such wells is the use of a conventional piston pump disposed at the bottom of the well. The pump is reciprocated by a string of rods which extend to the surface where they are reciprocated by a beam pumping unit. In order to ensure efficient operation of beam pumping units, it is necessary to periodically measure the energy input to the pump and compute its performance. The energy input to pump can be calculated by measuring both the load on the rod string and the displacement of the rod string. Using these figures and following the teachings of U.S. Pat. No. 3,343,409 one can determine the shape of the dynamometer card for the downhole pump. The shape of the dynamometer card will indicate the operating condition of the downhole pump.
A recently issued U.S. Pat. No. 3,951,209 describes a method for determining when a rod pumped well is pumped-off by measuring the load on the rod and displacement of the rod. This permits one to determine when the well has pumped-off and thus shut in the pump until additional fluid accumulates in the well. The term "pumped-off" is used to describe the condition wherein the bottom-hole pump does not completely fill with liquid on the upstroke and on the succeeding down-stroke the rod string falls free until the pump contacts the fluid. The free falling of the rod string and the sudden contacting of the fluid produces shock loads in the rod string which can damage both the downhole pump unit and the beam pumping unit at the surface. In addition, the operation of the pump in a pump-off condition is inefficient and wastes energy. For efficient operation the pumping unit should be stopped when the well is pumped-off and fluid allowed to accumulate in the well before restarting the pumping unit. Thus, using continuous monitoring equipment one can adjust the duty cycle of the pump to match the production of the well. This will result in both the maximum production and the lowest power consumption for operating the pumping unit.
The present monitoring systems use load cells for measuring the force on the rod and variable potentiometers to measure the displacement of the beam of the pumping unit or of the polished rod. The displacement of the beam of the pumping unit can, of course, be related to actual displacement of the rod string. The use of a variable potentiometer requires the movable arm to be aligned with and physically connected to the beam of the pumping unit or to the polished rod. Thus, the potentiometer cannot be completely sealed and the unit is open to atmospheric conditions. In addition, the continuous wiping movement of the arm across the potentiometer causes wear that necessitates their frequent replacement.